I would guess insecurity and jealously as well. I could understand if it was a single woman living with you or coming over but if it is the cousins girlfriend for example and that doesn't make sense to me. I get jealous myself because I worry that the man will cheat on me. But i have always been comfortable with other guys girls around my boyfriends. When you talked to her about it what did she say? She doesn't think she is jealous, so what does she think? Maybe she just doesn't like other women? some ladies are like that.